In the United States and Canada, this time zone is generically called Mountain Time (MT). Specifically, it is Mountain Standard Time (MST) when observing standard time (fall and winter), and Mountain Daylight Time (MDT) when observing daylight saving time (spring and summer). The term refers to the fact that the Rocky Mountains, which range from northwestern Canada to the US state of New Mexico, are located almost entirely in the time zone. In Mexico, this time zone is known as the Pacific Zone.

In some areas, starting in 2007, the local time changes from MST to MDT at 2am MST to 3am MDT on the second Sunday in March and returns at 2am MDT to 1am MST on the first Sunday in November.

Sonora in Mexico and most of Arizona in the United States do not observe daylight saving time, and during the spring, summer, and autumn months they are on the same time as Pacific Daylight Time.[4] The Navajo Nation, most of which lies within Arizona, observes daylight saving time, although the Hopi Nation, as well as some Arizona state offices lying within the Navajo Nation, do not.

Nevada – West Wendover, on the Utah border, is the only location in the state which legally observes Mountain Time.[5] However, the towns of Jackpot, Jarbidge, Mountain City and Owyhee, while all legally within the Pacific Time Zone, locally observe the Mountain Time Zone due to proximity to and stronger connections with towns in nearby Idaho.[6]

Also, the unincorporated community of Kenton, Oklahoma, located in the extreme western end of the Oklahoma Panhandle, unofficially observes Mountain Time (as the nearest sizable towns are located in Colorado and New Mexico, both of which are in the Mountain Time Zone). However, the entire state of Oklahoma is officially in the Central Time Zone. Additionally, western Culberson County, Texas unofficially observes Mountain Time.

^The specification for the Mountain Time Zone in the United States is set forth at 49 CFR 71.8.[1] The boundary between Central and Mountain time zones is set forth at 49 CFR 71.7,[2] and the boundary between Mountain and Pacific time zones is set forth at 49 CFR 71.9.[3]

^Delen Goldberg (July 2, 2011). "Nevada’s tiny town with a different time zone". Las Vegas Sun. Retrieved 2014-07-03. West Wendover is the only town in Nevada that runs on Mountain Time. Jackpot, an even smaller town in Elko County, unofficially observes Mountain Time but is technically part of the Pacific Time Zone. (quote in slide 4)